Polymer-supported crown ethers catalytic activity

Polymer-supported crown ethers and cryptands were found to catalyze liquid-liquid phase transfer reactions in 1976 55). Several reports have been published on the synthesis and catalytic activity of polymer-supported multidentate macrocycles. However, few studies on mechanisms of catalysis by polymer-supported macrocycles have been carried out, and all of the experimental parameters that affect catalytic activity under triphase conditions are not known at this time. Polymer-supported macrocycle... 

Polymer-supported onium ions are relatively unstable under severe conditions, especially concentrated alkali154). Polymer-supported crown ethers and cryptands are stable under such conditions. In practice, they could be reused without loss of catalytic activity for the alkylation of ketones under basic conditions, whereas the activity of polymer-supported ammonium ion 7 decreased by a factor of 3 after two recycles of the catalyst147). 

With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

Phase-transfer catalysts, such as the classic onium salts, crown ethers, and cryptands, have been immobilized on insoluble polymer matrices with various degrees of cross-linking. Their activity remains reasonably high if the catalytic centre is sufficiently far from the polymer backbone or if the resin is very porous. However, with phosphonium salts immobilized on silica gel die length of the hydrophobic chain between the active centre and the matrix and the solvent determine the adsorption capacity of the polar support, which then controls the rate of reaction. ... 

Quaternary ammonium and phosphonium ions bound to insoluble polystyrene present an even more complicated mechanistic problem. Polystyrene beads lacking onium ions (or crown ethers, cryptands, or other polar functional groups) have no catalytic activity. The onium ions are distributed throughout the polymer matrix in most catalysts. The reactive anion must be transferred from the aqueous phase to the polymer, where it exists as the counter ion in an anion exchange resin, and the organic reactant must be transferred from the external organic phase into the polymer to meet the anion. In principle, catalysis could occur only at the surface of the polymer beads, but kinetic evidence supports catalysis within the beads for most nucleophilic displacement reactions and for alkylation of phenylacetonitrile. 

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